[OpenMP][test] fix omp_get_wtime.c test to be more accommodating

The omp_get_wtime.c test fails intermittently if the recorded times are
off by too much which can happen when many tests are run in parallel.

Instead of failing if one timing is a little off, take average of 100
timings minus the 10 worst.

Differential Revision: https://reviews.llvm.org/D108488